Hitherto, a solid state relay circuit which switches an A.C. load current responsive to an input signal to the input circuit electrically isolated from the load circuit and is provided with a zero-cross firing means has been known, for instance, by the U.S. Pat. No. 3,723,769 on the invention of Collins, et al. assigned to International Rectifier Corporation or by the U.S. Pat. No. 3,648,075 on the invention of Mankovitz assigned to Teledyne Incorporated. In these known solid state relays, it was necessary to provide full-wave rectifier circuits to feed rectified full-wave currents to thyristors or triacs. Moreover, each of these circuits has one thyristor or triac, which is connected in a manner to be switched "on" at every rising-up of both positive and negative parts of the load current, and is for regulating the zero-cross firing operation. The present inventor found a fact that, thyristors or triacs are liable to accidental turning-ons even without input signal at the gate. Such accidental operation is such that when a ripple current obtained by means of the full-wave rectifier is applied to its gate, the thyristor or the triac is liable to an accidental turning-ons even at small noises in the anode-cathode voltage thereof or at an excessive dV/dt value exceeding the so-called critical off-voltage-rise-up ratio of the anode-cathode voltage.